The present invention relates to a magnetic field measurement technique employing a SQUID (Superconducting Quantum Interference Device) magnetometer, which is a superconducting device. More particularly, it relates to a technique for reducing an interference magnetic field in the magnetic field measurement apparatus.
In the measurement by the magnetic field measurement apparatus employing the SQUID, a measurement such as extremely feeble magnetoencephalogram and magnetocardiogram is performed in the interior of a magnetically shielded room having an extinction ratio of 40 to 50 dB or more. In many cases, a pick-up coil for picking up a biomagnetic field has a form of first-order gradiometer, which picks up a difference between a one-turn pick-up coil and a coil inversely wound. The first-order gradiometer features that it cancels the interference magnetic field generated from a magnetic field source located faraway, and it is capable of picking up a signal as to the magnetic field generated from the vicinity such as the heart and the brain without canceling in large scale. Therefore, an impact from the interference magnetic field can be easily reduced. Usually, the first-order gradiometer brings an attenuation of approximately 40 to 50 dB with respect to a uniform field.
As thus described, it is possible to cancel the interference magnetic field at least approximately 80 to 100 dB by combining the magnetically shielded room and the first-order gradiometer. However, in the case where an object generating a large magnetic field, such as a train and a car, passes nearby at a distance of around 50 m to 100 m from the magnetically shielded room, there is observed an interference magnetic field larger than the magnetic field generated from a living body by a factor of 10 times. In order to cancel such intense interference magnetic field, various methods have been attempted so far.
For example, as shown in FIG. 9, there has been proposed a method where a magnetic field signal is used, which is picked up from a magnetometer sensor unit 15 of a fluxgate that is positioned outside the magnetically shielded room 16, allowing a feedback current to flow into a solenoid coil 8 wound on the magnetically shielded room externally, thereby conducting an adjustment so that an output from the fluxgate becomes zero (For example, see H J M ter Brake et al.: “Improvement of the performance of a i-metal magnetically shielded room by means of active compensation”, Meas. Sci. Technol. (1991), Vol.2, p.596–601). The solenoid coil 8 is arranged in such a manner as going along the exterior side surface of the magnetically shielded room 16.
The solenoid coil 8 is connected to a magnetometer control unit 14, which converts a magnetic field signal obtained from the magnetometer sensor unit 15 to a voltage, a PID control circuit 21, and a current converter 10. The magnetometer control unit 14 converts the interference magnetic field picked up by the magnetometer sensor unit 15 to a voltage, the PID control circuit 21 adjusts the current converted by the current converter 10, and allows thus adjusted current to flow into the solenoid coil 8. The adjustment of the current amount by the PID control circuit 21 is carried out so that the output from the magnetometer control unit 14 is minimized, or the output of the interference magnetic field from the magnetometer control unit 14 is minimized.
Since there is a shielding factor distortion spatially in the magnetically shielded room 16, it has been considered that the interference magnetic field picked up outside the magnetically shielded room 16 can be canceled with the highest degree of accuracy by using a canceling magnetic field, which is generated by the solenoid coil 8 positioned outside the magnetically shielded room 16.
In addition, there has been proposed a softwarily canceling method by use of a signal from a reference coil employing a SQUID sensor (for example, see J Vrba et al.: “Biomagnetometer for unshielded and well shielded Environments”, Clin. Phys. Physiol. Meas. (1991) Vol.12 Suppl. B, p.81–86). However, details of the reference coil have not been described.
With the conventional method by use of the solenoid coil as described above, a canceling coil positioned outside the magnetically shielded room generates a uniform field within the magnetically shielded room. Therefore, in a multi-channel system including a plurality of first-order gradiometers, it is hard to correct variations in canceling rates with respect to each of the first-order gradiometers in respective channels.
In the softwarily canceling method as described above, there is no disclosure as to a configuration of the reference coil. Further, the cancel data being described is limited to those obtained without using the magnetically shielded room, and thus there is no discussion about a problem of distortion due to the magnetically shielded room.